1. Field of the Invention
The present invention relates to a Moineau type progressive cavity positive displacement downhole drilling motor that uses high pressure fluid to drive the rotor.
More specifically, it is a means to improve the life expectancy of the motor's elastomeric stator and provide conduits for data transmission.
2. Description of the Prior Art
A state of the art positive displacement progresive cavity drilling motor is constructed with an outer steel tubular housing with an elastomeric liner (usually a nitrile type rubber) vulcanized and bonded to the inside diameter of the tubular housing. A through hole is formed in the center of the elastomeric liner having a multiplicity of essentially semi-circular profiled convoluted lobes. This forms the stator for the mating convoluted steel rotor, which has one less lobe than the aforesaid stator. The number of lobes on the rotor/stator power section is predicated on the desired speed of revolution of the rotor.
It is evident, for example, that this construction, as shown in U.S. Pat. No. 2,085,115, creates a great variance in the cross sectional thickness of the elastomer between the lobes and the valleys separating them. This causes a large variance of the elastomeric (rubber) properties particularly in the wide and narrow sections. The cured elastomer properties (i.e. hardness, compression set, elastic modulus and other properties) are a time/temperature function. A thin or narrow section reaches maximum curing temperature more quickly and stays at this temperature longer than the thicker or wider sections, thereby curing the thin sections to a greater degree. Therefore, the thin elastomeric sections have much different physical properties than the thick sections. In operation of the fluid or "mud" motor in an earthen formation, as the steel rotor is forced to rotate inside the elastomeric stator with non-uniform physical properties, the elastomer is subjected to an extremely high level of cyclic stress reversals.
The hysteresis that is inherent under the above conditions creates a large amount of heat that adds to the degradation of the elastomer. The elastomer reaches a limit in tensile strength and the high shear and tensile stresses imposed by the spinning helical rotor tears through the embrittled sections and large pieces are ripped out. This phenomenon is known as "chunking" in the drilling industry. Obiviously, chunking of the elastomer destroys the usefulness of the drilling motor.
Another limitation in all the prior art is the lack of a much needed means to directly transmit, through the motor, data generated by sensors at or near the drilling bit at the hole bottom to electronic processors located in the drill string above the downhole motor. This data, after processing, is transmitted to the surface by Measurement While Drilling (MWD) or other transmission systems.
Typical examples of the prior art, U.S. Pat. Nos. 3,840,080, 3,982,858, 4,059,165 and 4,646,856 all depict elastomeric stators with non-uniform cross-sections and none have means for electronic data transmission through the downhole motor. Therefore, all of the known prior art have elastomer problems and data transmissions limitations heretofore described